Restrict Background Fetch API when called from a Service Worker

This policy controls whether background fetch requests from Service Workers are restricted. If a feature that downloads files in the background is affected, this policy may be relevant. If this policy is set to Enabled, or left unset, the restriction is active, and background fetch requests from Service Worker contexts may be blocked. If this policy is set to Disabled, the restriction is bypassed, allowing all Service Workers to make background fetch requests. This enterprise policy is temporary, and will be removed after M152.